Method for controlling solute loading of polymer microparticles

ABSTRACT

Solute-loaded polymer microparticles are obtained by immersing microparticles in a bath comprising a selected solute dissolved in a ternary solvent system. A first solvent of the ternary system is a strong solvent for both the solute and the polymer from which the microparticle was formed. A second solvent is a weak solvent or non-solvent for the solute and the polymer (tuning solvent). A third solvent is a weak solvent or non-solvent for the solute and polymer, but serves as a co-solvent with respect to the first and second solvents in that it is miscible with both the first and second solvents. The amount of solute incorporated into the microparticles is controlled by adjusting the ratio of solute with respect to the microparticle polymer, and by adjusting the composition of the ternary solvent system, principally the amount of tuning solvent. The method is particularly useful for providing libraries of combinatorially encoded microparticles containing distinguishable dye loadings, particularly distinguishable fluorescent dye loadings.

FIELD OF THE INVENTION

The invention relates to a method for controlling the partitioning of asolute between a liquid continuous phase and a solid dispersed phase,such as in the production of stained particles.

BACKGROUND OF THE INVENTION

Polymer particles containing an entrained solute, e.g., dye, are widelyused as markers for biomolecules and as internal reference andcalibration standards for assay detection methods such as flowcytometry. Four general methods have been described in the prior art forproducing fluorescent polymer particles: (A) copolymerization of dye andmonomer; (B) partitioning of water-soluble or oil-soluble dyes intopreformed particles; (C) surface functionalization of preformedparticles; and (D) encapsulation of dye droplets. In addition,polymerization methods also have been used to prepare core-shellparticles, that is, microparticles comprised of a polymer core and apolymer shell.

A. Copolymerization Based Methods

Fluorescent microparticles may be synthesized by polymerization ofmonomeric units to form microparticles in the presence of fluorescentdyes. U.S. Pat. No. 4,326,008 to Rembaum (1982) describes the synthesisof fluorescent microparticles by copolymerization of functionalizedacrylic monomer with a polymerizable fluorescent comonomer. The methodgenerally requires a polymerizable dye molecule. Such methods, generallysuffer from the drawback of possible inhibition of polymerization by thefluorescent dye and/or bleaching of the fluorescence by the reactiveconstituents of the polymerization reaction.

U.S. Pat. No. 4,267,235 to Rembaum (1981) describes the synthesis ofpolygluteraldehyde microspheres using suspension polymerization.Cosolubilized fluorescein isothiocyanate (FITC) is used to createfluorescent microspheres. Suspension condensation polymerization of themonomer with cosolubilized dye molecules, while largely circumventingdye destruction and polymerization inhibition, generates a broadparticle size distribution and hence is not a suitable route for theproduction of monodisperse fluorescent microspheres.

U.S. Pat. No. 5,073,498 to Schwartz et al. (1991) describes a processfor making fluorescent microparticles by seeded polymerization. One ormore hydrophobic fluorescent dyes are dissolved in a solution containingmonomer and initiator. The solution is added to pre-swollenmicroparticles. The patent discloses methods permitting the introductionof three different dyes into a particle. The method suffers from thedrawback of possible inhibition of polymerization by the fluorescentdye, or conversely the bleaching of the fluorescence by thepolymerization process.

Multi-stage emulsion polymerization has been employed to preparecore-shell particles without surface functional groups. U.S. Pat. No.5,952,131 to Kumaceheva et al. discloses a method for preparing stainedcore-shell particles. The method is based on multiple stages ofsemi-continuous polymerization of a mixture of two monomers (methylmethacrylate and ethylene glycol dimethacrylate) and a fluorescent dye(4-amino-7-nitrobnezo-2-oxa-1,3 diazol-labeled methyl methacrylate). Theparticles are then encapsulated with an outer shell by copolymerizationof methyl methacrylate and butylmethacrylate in the presence of chaintransfer agent, dodecyl mercaptan. Kumaceheva et al. do not prepare anddo not have as an object the inclusion of surface functional groupcore-shell polymer product.

U.S. Pat. No. 4,613,559 to Ober et al. discloses a method for preparingcolored toner by swelling. Polystyrene particles (5.5 micron) areprepared by dispersion polymerization of styrene in the presence ofethanol, poly(acrylic acid), methylcellosolve and benzoyl peroxide.Swelling is performed by dispersing the polystyrene in an aqueoussolution of sodium dodecyl sulfate and acetone. Colored particles areobtained by adding an emulsified dye solution (Passaic Oil Red 2144 inmethylene chloride emulsified with an aqueous solution of sodiumdodecylsulfate) to the particle dispersion.

Polymerization methods have been employed to prepare core-shellparticles containing surface functional groups. U.S. Pat. No. 5,395,688to Wang et al. discloses magnetically-responsive fluorescent polymerparticles comprising a polymeric core coated with a layer of polymercontaining magnetically-responsive metal oxide. The final polymer shellis synthesized with a functional monomer to facilitate covalent couplingwith biological materials. The procedure of Wang et al. is based onthree steps: (1) preparation of fluorescent core particles; (2)encapsulation of metal oxide in a polystyrene shell formed over thefluorescent core by free radical polymerization in the absence ofemulsifier but with an excess of initiator; and (3) coating of themagnetic fluorescent particles with a layer of functional polymer. Thefunctional polymer has carboxyl, amino, hydroxy or sulfonic groups. Wanget al. do not describe a method for obtaining the colored core and alsodoes not address the problem of destruction of dye during the freeradical polymerization process.

U.S. Pat. No. 4,829,101 to Kraemer et al. discloses two-micronfluorescent particles obtained by core-shell polymerization. The core isobtained at 80° C. by polymerizing a mixture of isobutyl methacrylate,methyl methacrylate and ethylene glycol dimethacrylate via ammoniumpersulfate initiation. A shell is synthesized over the core bysemi-continuously adding, in a first step, a mixture of the samemonomers containing a fluorescent dye (fluoro-green-gold). Through theend of the reaction, two different monomer mixtures are added over a onehour period: a first mixture containing methyl methacrylate, ethyleneglycol-bis-(methacrylate) and glycidyl methacrylate, and a secondmixture containing methacrylamide and initiator. The polymerization isinitiated with 4,4′-azobis-(cyanovaleric acid).

Okubo et al., Colloid Polym. Sci. 269:222-226 (1991), Yamashita, et al.,Colloids and Surfaces A., 153:153-159 (1999), and U.S. Pat. No.4,996,265 describe production of micron-sized monodispersed polymerparticles by seeded dispersion polymerization. Polymer seed particlesare pre-swelled with large amounts of monomer prior to seededpolymerization. The swelling is carried out by slow, continuous,drop-wise addition of water to an ethanol-water mixture containing theseed particles, monomers, stabilizer and initiator. The addition ofwater decreases the solubility of the monomer in the continuous phase,leading to precipitation and subsequent absorption of monomer onto orinto the seed polymer particles. The monomer absorbed into the seedpolymer particle is then polymerized to produce large monodispersedpolymer particles.

B. Partitioning of Water-Soluble or Oil-Soluble Dyes

Fluorescent particles can be produced by permitting dye molecules topartition into pre-swollen microparticles according to a techniqueoriginally described by L. B. Bangs (Uniform Latex Particles; SeragenDiagnostics Inc., 1984, p. 40). The process involves dissolution of adye molecule or mixture of dye molecules in a solvent or solvent mixtureof choice containing polymer microparticles. Absorption of the solventby the microparticles leads to swelling, permitting the microparticlesto absorb a portion of the dye present in the solvent mixture. Thestaining process is usually terminated by removing the solvent. Thelevel of dye partitioning is controlled by adjusting the dyeconcentration, and in the case of a plurality of dyes, the relativeabundance of individual dyes. Microparticles stained in this manner arequite stable and uniform. However, in many cases, depending on thechoice of solvent system, a large dye excess is required to attain thedesired partitioning, leading to significant loss of expensive dyematerial.

U.S. Pat. No. 5,723,218 to Haugland et al. (1998), U.S. Pat. No.5,786,219 to Zhang et al. (1998), U.S. Pat. No. 5,326,692 to Brinkley etal. (1994) and U.S. Pat. No. 5,573,909 to Singer et al. (1996) describeprotocols for producing various fluorescently-colored particles byswelling and dye partitioning in organic solvent and organic solventmixtures. Various types of fluorescent particles, for example,fluorescent particles containing multiple dyes, particles exhibitingcontrollable and enhanced Stokes shifts, and particles displayingspherical zones of fluorescence, are described.

International patent application WO 99/19515 of Chandler et al. (1997)describes an improved method for the production of a series ofratiometrically-encoded microspheres with two dyes. A protocol for theproduction of 64 different encoded microspheres is reported. A swellingbath composition using a mixture of an organic solvent and alcohol(under anhydrous conditions) also is disclosed.

U.S. Pat. No. 5,266,497 to Matsudo et al. (1993) describes a method forgenerating a dye-labeled polymer particle which uses a hydrophobic dyedissolved in an organic solvent emulsified in water. The dyed particleswere used for immuno-chromatographic purposes.

U.S. Pat. No. 4,613,559 to Ober et al. (1986) describes the synthesis ofcolored polymer particles using oil-soluble dyes. The disclosed methoduses an emulsion of a dichloromethane dye solution in a water andacetone mixture for coloring the particles.

C. Functionalization of Internal or External Microparticle Surfaces

Production of fluorescent particles by surface functionalizationinvolves the covalent attachment of one or more dyes to reactive groupson the surface of a preformed microparticle. This leaves the dyemolecules exposed to the environment, which can hasten the decompositionof the dye. In addition, surface functionalization often renders aparticle surface very hydrophobic, inviting undesirable non-specificadsorption and, in some cases, loss of activity of biomolecules placedon or near the particle surface. These problems can be circumvented byattaching a stained small particle, in lieu of a dye molecule, to thesurface of a carrier particle. The efficacy of this method in generatinglarge sets of encoded particles from a small number of dyes (ratioencoding) is unclear.

U.S. Pat. No. 4,487,855 to Shih (1984), U.S. Pat. No. 5,194,300 toCheung (1993) and U.S. Pat. No. 4,774,189 to Schwartz (1988) disclosemethods for preparation of colored or fluorescent microspheres bycovalent attachment of either one or a plurality of dyes to reactivegroups on the preformed particle surface. Battersby et al., “TowardLarger Chemical Libraries: Encoding with Fluorescent Colloids inCombinatorial Chemistry” J. Am. Chem. Soc. 2000, 122, 2138-2139;Grondahl et al., “Encoding Combinatorial Libraries: A Novel Applicationof Fluorescent Silica Colloids”, Langmuir 2000, 16, 9709-9715; and U.S.Pat. No. 6,268,222 to Chandler et al. (2001) describe a method ofproducing fluorescent microspheres by attaching to the surface of acarrier microparticle a set of smaller polymeric particles that arestained.

D. Encapsulation Methods

Formation of fluorescent particles by encapsulation utilizes a solutionof a preformed polymer and one or more dyes. In one approach, thesolution is dispensed in the form of a droplet using a vibrating nozzleor jet, and the solvent is removed to produce polymer particlesencapsulating the dye. This process requires specialized processequipment and displays only limited throughput. Alternatively, a polymerdye mixture is emulsified in a high-boiling solvent and the solution isevaporated to yield polymer-encapsulated dye particles. This processoften generates non-spherical particles with broad size distribution.

U.S. Pat. No. 3,790,492 to Fulwyler et al (1974) discloses a method toproduce uniform fluorescent microspheres from a pre-dissolved polymerand dye solution using a jet. U.S. Pat. No. 4,717,655 to Fulwyler et al.(1988) discloses a process which includes two dyes in pre-designatedratios in a polymer microparticle to produce five distinguishabletwo-color particles.

The various prior art methods of producing fluorescent microparticlessuffer from certain disadvantages. Where strong swelling solvents areused, the microparticles must be cross-linked to prevent them fromdisintegrating and deforming in the dye solution. This constraintrepresents a severe limitation since the majority of dyes require fortheir dissolution at any reasonable concentration solvent systems inwhich most polymer particles of interest, notably polystyrene particles,also will dissolve. These considerations have restricted the applicationof solvent swelling in the prior art to chemically stabilized(“cross-linked”) microparticles. This restriction introduces additionaldifficulty and cost in microparticle synthesis; highly cross-linkedparticles are often very difficult to synthesize. Also, restriction tocross-linked particles limits the degree of microparticle swelling andthus the degree of dye incorporation. Specifically, the application ofsolvent swelling protocols of the prior art conducted on cross-linkedmicroparticles generally limits penetration of the dye to the outerlayer of the microparticle, thereby precluding uniform staining of theentire interior volume of individual particles and generally alsoprecluding the realization of high levels of dye incorporation. What isneeded is a process that can utilize non-cross-linked, as well ascross-linked, particles. What is needed is a method that will providedye-loaded non-cross-linked polymer microparticles, which may be used,for example, to prepare libraries of dyed microparticles havingcontaining different dyes and/or different dye amounts.

The degree of particle swelling in prior art solvent swelling-basedmethods of dye incorporation determines the rate of dye transport intothe particles. Diffusion barriers lead to non-uniform dye distributionin the microparticles. For this reason, intense micro-mixing (broughtabout by either efficient mechanical mixing or by sonication) isrequired in order to produce uniformly stained populations ofmicroparticles. These vigorous mixing procedures, while effective forlaboratory scale preparation, are not easily adapted to larger scales.For example, sonication often requires specialized equipment such asprobe sonicators, and limits the parallel completion of multiplestaining reactions. What is needed is a dyed particle manufacturingprocess that requires less vigorous mixing or no mixing, and permitsparallel staining reactions to be performed.

Microparticles stained by prior art swelling methods are vulnerable tosubsequent exposure to solvents that may cause substantial loss of dyeand may preclude the implementation of protocols providing for multiplesequential dye incorporation steps.

In the prior art methods, the degree of dye partitioning into thepolymer matrix is controlled by explicit variation of the initialconcentration of dye in the dye solution. This approach, whilepermitting the realization of multiple, distinct levels of dyeinclusion, suffers from a number of disadvantages. For example, highlevels of staining frequently are not attainable because of the limitedsolubility of the dye in the bath. Even when solubility is not an issue,the low partition coefficients of many dyes requires a large excess ofdye in solution introducing the risk of deleterious effects onsubsequent bioanalytical assays. In fact, when carboxylate-modifiedbeads are prepared by prior art solvent-swelling methods, the carboxylfunction may become inoperative, and may be no longer available forfunctionalization by covalent coupling to other chemical groups. Inaddition, valuable dye material is lost in significant quantities. Whatis needed is a process for preparing stained microparticles, andfluorescent microparticles in particular, that achieves dyeincorporation even from poorly soluble dye/solvent formulations. What isneeded is a process that allows for precise control of the solute (dye)loading level in polymer microparticles during the staining process.

SUMMARY OF THE INVENTION

According to one embodiment, a method for modulating the loading of asolute in polymer microparticles is provided. The method comprises:

(a) providing:

-   -   (i) at least one first solvent in which the solute and the        microparticle polymer are soluble;    -   (ii) at least one second solvent in which the solute and the        microparticle polymer are not or only weakly soluble, said first        and second solvents being immiscible or at most partially        miscible;    -   (iii) at least one third solvent in which the solute and the        microparticle polymer are not or only weakly soluble, said third        solvent being miscible with the first and second solvents;

(b) forming a suspension of said polymer microparticles in a designatedvolume of a mixture comprising at least one second solvent and at leastone third solvent;

(c) adding to said polymer microparticle suspension a solutioncomprising a solute dissolved in said first solvent whereby the soluteis taken up by the microparticles to provide a suspension ofmicroparticles characterized by a desired concentration of said solutein the microparticles;

(d) changing the concentration of said solute in at least a portion ofthe microparticles to a selected second solute concentration by addingto the microparticle suspension or fraction thereof a selected amount ofthe solute, a selected amount of at least one second solvent, orcombination of solute and second solvent, whereby less than completepartitioning of the solute from the suspension liquid phase to themicroparticles takes place.

According to one embodiment, the selected second solute concentration isachieved by adding to the suspension of microparticles characterized bythe first solute loading concentration a selected amount of at least onesecond solvent. According to another embodiment, a plurality ofmicroparticle fractions of selected solute concentrations are providedby dividing the suspension of microparticles characterized by the firstsolute concentration into fractions, and adding selected amounts of asecond solvent to the fractions.

According to another embodiment of the invention, a method formodulating the solute loading of polymer microparticles comprises:

(a) providing:

-   -   (i) at least one first solvent in which the solute and the        microparticle polymer are soluble;    -   (ii) at least one second solvent in which the solute and the        microparticle polymer are not or only weakly soluble, said first        and second solvents being immiscible or at most partially        miscible;    -   (iii) providing at least one third solvent in which the solute        and the microparticle polymer are not or only weakly soluble,        said third solvent being miscible with the first and second        solvents;

(b) forming a suspension of said polymer microparticles in a designatedvolume of a mixture comprising at least one second solvent and at leastone third solvent;

(c) adding to said polymer microparticle suspension a solution of solutedissolved in said first solvent whereby the solute is taken up by themicroparticles to provide a suspension of microparticles characterizedby a desired first concentration of the solute;

(d) continuously or semi-continuously adding second solvent to themicroparticle suspension to continuously or semi-continuously modulatethe microparticle solute concentration;

(e) removing at least one portion of said microparticles from thesuspension at a time interval during the course of said second solventaddition to provide at least two microparticle sets which differ insolute concentration.

In another embodiment, the invention is a method for modulating the dyeloading of polymer microparticles comprising:

(a) providing microparticles characterized by a first concentration ofat least one dye contained in said microparticles, said microparticlessuspended in a dye solution comprising the at least one dye and asolvent system comprising:

-   -   (i) at least one first solvent in which the dye and the        microparticle polymer are soluble;    -   (ii) at least one second solvent in which the dye and the        polymer are not or only weakly soluble, said first and second        solvents being immiscible or at most partially miscible;    -   (iii) at least one third solvent in which the dye and the        polymer are not or only weakly soluble, said third solvent being        miscible with the first and second solvents;

(b) adding to said microparticle suspension a selected amount of thesecond solvent to change the amount of the dye partitioning to thepolymer microparticles and the concentration of dye in saidmicroparticles; and

(c) incubating the microparticle suspension for a period of time so thatthe amount of dye partitioning to the microparticles, for a giveninitial dye concentration in the dye solution, is determined by theamount of second solvent added to the microparticle suspension.

In yet another embodiment, the invention is a method of producing dyedpolymer microparticles comprising:

(a) providing:

-   -   (i) at least one first solvent in which the dye and the        microparticle polymer are soluble;    -   (ii) at least one second solvent in which the dye and the        microparticle polymer are not or only weakly soluble, said first        and second solvents being immiscible or at most partially        miscible;    -   (iii) at least one third solvent in which the dye and the        microparticle polymer are not or only weakly soluble, said third        solvent being miscible with the first and second solvents;

(b) forming a suspension of said polymer microparticles in a designatedvolume of a mixture comprising at least one second solvent and at leastone third solvent;

(c) adding to said polymer microparticle suspension a solutioncomprising the dye dissolved in said first solvent whereby the dye istaken up by the microparticles to provide a master-batch suspension ofmicroparticles characterized by a first concentration of said dye in themicroparticles;

(d) creating two or more aliquots from said microparticle master-batchsuspension containing selected added amounts of second solvent to changethe amount of dye partitioning to the polymer microparticles in saidaliquots; and

(e) incubating the microparticle suspension aliquots for a period oftime so that the amount of dye partitioning to the microparticles, for agiven initial dye concentration in the dye solution, is determined bythe amount of second solvent added to the microparticle suspensionaliquots.

An automated method for producing dyed polymer microparticles is alsoprovided. The method comprises:

(a) providing a microparticle master-batch suspension comprisingmicroparticles characterized by a first dye state, said microparticlessuspended in a dye solution comprising at least one dye and a solventsystem comprising:

-   -   (i) at least one first solvent in which the dye and the        microparticle polymer are soluble;    -   (ii) at least one second solvent in which the dye and the        polymer are not or only weakly soluble, said first and second        solvents being immiscible or at most partially miscible;    -   (iii) at least one third solvent in which the dye and the        polymer are not or only weakly soluble, said third solvent being        miscible with the first and second solvents;

(b) creating two or more microparticle suspension aliquots from saidmaster-batch suspension, each such suspension aliquot characterized bymicroparticles of said first dye state suspended in said dye solution;

(c) executing, at least once for each created aliquot, the followingsequence of steps to transform the microparticle dye state in eachaliquot from said first dye state to a selected second dye state:

(i) computing, for the selected second dye state:

-   -   -   (1) the amount of dye dissolved in said first solvent, and        -   (2) the amount of second solvent, required to be added to            said aliquot to attain said selected second microparticle            dye state; and

    -   (ii) dispensing to said aliquot the amount of dye dissolved in        said first solvent and the amount of second solvent required to        attain said selected second microparticle dye state.

An apparatus for producing dyed polymer microparticles comprises acomputer operatively connected to a pipetting robot, wherein thecomputer is programmed to carry out the aforesaid automated method

The step of creating said two or more microparticle suspension aliquotscontaining selected added amounts of second solvent may comprisedividing the microparticle suspension master-batch into two or morealiquots, and adding selected amounts of second solvent to saidaliquots. Alternatively, the step of creating the two or moremicroparticle suspension aliquots containing selected added amounts ofsecond solvent may comprise continuously or semi-continuously addingsecond solvent to the microparticle suspension master-batch and removingat least one portion of the master-batch at a time interval during thecourse of the second solvent addition. The result is the formation oftwo or more microparticle suspension aliquots containing selected addedamounts of second solvent.

DESCRIPTION OF THE FIGURES

FIG. 1( a) is a schematic representation of a ternary solvent solutionfor use in the present invention.

FIG. 1( b) is a diagram of the sequence of steps of an embodiment of theinvention.

FIG. 2 is a three-dimensional plot of the level of dye incorporationinto polymer microparticles in a solvent bath according to the presentinvention as a function of three variables. (X) is the mass of dye inthe bath (represented as the concentration of the dye, C^((s)),multiplied by the volume of the dye added to the bath, V^((s))) dividedby the volume of the microparticles, V^((p)). (Y) is the volume fractionof the microparticles in the bath, φ^((p)), divided by the volumefraction of solvent, φ^((s)). (Z) is the concentration of dye containedin the microparticles, C^((p)). Thus, the (Z) axis represents the massof dye partitioned into the microparticle divided by the microparticlevolume. The volume fraction of the particles, φ^((p)), is given by theequation φ^((p))=(1−φ^((s))). Line (P₀) in plane (X,Z) represents dyepartitioning as a function of X into microparticles in the absence oftuning solvent. Line (P₁) represents dye partitioning intomicroparticles as a function of the mass of dye in the bath (X) in thepresence of a volume fraction, φ, of tuning solvent.

FIG. 3( a) is a schematic diagram of a parallel processing embodiment ofthe invention for producing n sub-populations (F_(n)(S_(n))) offluorescently stained microparticles, from n solventsystem/microparticle suspensions B_(n). Each suspension contains adesignated amount Sn of tuning solvent.

FIG. 3( b) is a schematic diagram of a serial processing embodiment ofthe invention for producing n sub-populations of fluorescently stainedmicroparticles from a single reaction. [B] Represents a pre-incubatedmaster-batch of solvent system/microparticle suspension, to which acontinuous stream of tuning solvent is fed. F_(n)(S_(n)) representfractions of the microparticle suspension removed from the master-batchat times t=t_(Fn). The fractions contain S_(n) amounts of tuningsolvent.

FIG. 3( c) is a schematic diagram of an embodiment of the invention forproducing n sub-populations F_(n)(S_(n)) of fluorescently stainedmicroparticles by a combination of series and parallel processing. [B]Represents a pre-incubated master-batch of solvent system/microparticlesuspension to which a known amount of tuning solvent, S(t_(F)), is fed.The master-batch is then split into n different aliquots, B_(n), towhich n different designated amounts of tuning solvent, δS_(n), areadded.

FIG. 3( d) is a schematic diagram of an embodiment of the invention forproducing m×n sub-populations F_(m)(S_(m), D_(n)) of fluorescentlystained microparticles by serial followed by parallel processing, usinga combination of solvent tuning and dye addition. S_(m) denotes theamounts of tuning solvent, and D_(n) the amounts of fluorescent dye,added to the various sub-populations F_(m)(S_(m), D_(n)).

FIG. 4 is a plot of the fluorescence of the collection of particlesprepared according to Example 1, below.

FIG. 5 is a plot of the fluorescence of the collection of particlesprepared according to Example 2, below.

FIG. 6( a) is a fluorescence calibration curve plotting the intensity offluorescence at emission wavelength 512 nm versus dye concentration forthe green fluorescent dye4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-pentanoicacid, succinimidyl ester, dissolved in ethanol.

FIG. 6( b) is a plot of the variation of the calculated amount of greenfluorescent dye entrapped in particles as a function of the water volumefraction utilized in preparing the particles of Examples 3-6.

FIG. 6( c) is a plot of the variation of the calculated amount of greenfluorescent dye entrapped in particles as a function of the greenfluorescent dye intensity for the particles of Examples 3-6.

FIG. 6( d) is plot of the variation of the partition coefficient of thegreen fluorescent dye as a function of Y (where, Y=(1−φ_(s))/φ_(s)).

FIG. 7 is a plot of the fluorescence of the collection of particlesprepared according to Example 26.

FIG. 8 is a plot of the fluorescence emitted by the particles producedaccording to Example 30.

FIG. 9 is a plot of the fluorescence emitted by the particles producedaccording to Example 31.

DETAILED DESCRIPTION OF THE INVENTION

The distribution of a solute between two immiscible phases designated 1and 2 is governed by the partition coefficient, K, which represents theratio of the equilibrium amounts of solute (N) in the two phases, thatis, K=[N₁/N₂].

The present invention provides methods to tune the partitioncoefficient, K, governing the distribution of a solute between twosubstantially immiscible phases, that is, distribution between a liquidcontinuous phase and a dispersed solid phase comprising solid particlesdispersed in the liquid continuous phase. Thus, the invention providesfor the controlled introduction of solutes into polymer microparticlesin solvent systems. The solute may comprise any material which may bepartitioned to the polymer microparticle phase, such as dyes, pigments,drugs, catalysts, nanoparticles, or any other useful material for whichloading onto or into a polymer microparticle is desired.

In a preferred embodiment, the solute is a dye, and the invention isillustrated hereinafter by the partition of dye (solute), between twosubstantially immiscible phases, namely a population of preformedpolymer microparticle (phase 1) and a homogeneous ternary solventmixture (phase 2). The microparticles comprise a solid phase. Thesolvent mixture comprises a liquid phase. The dye may be any substancecapable of imparting a desired color or desired fluorescence to apolymer microparticle. Thus, the dye may comprise a chromophore or afluorophore. The amount of dye incorporated in the microparticles isprecisely controlled by tuning, for given initial dye concentration, thecomposition of the ternary solvent mixture. As illustrated below, themethod is capable of producing populations of distinguishabledye-stained microparticles containing reproducible pre-determined levelsof dye, i.e. “dye encoding”, with minimal intra-sample variation of dyecontent.

The method of the present invention thus introduces solvent compositionas a novel control parameter for the preparation of stainedmicroparticles by providing for the quantitative adjustment, over a widerange, of the partition coefficient governing the distribution of dyebetween dye bath and microparticles. Specifically, the partitioncoefficient, K, of the dye under solvent tuning obeys the followingrelation: K=a exp(bY), where Y=(1−φ_(s))/φ_(s); φ_(s) denotes the volumefraction of the solvent; and (a) and (b) are constants.

Dye encoding according to the present invention results from varying thedye loading of the particles. By “loading” with respect to the dyecontained in a microparticle is meant the amount and/or character of thedye incorporated into the microparticle. The loading can thus vary by atleast one property selected from the (i) amount of incorporated dye and(ii) the identity of incorporated dye. Encoding may thus take the formof varying the amount of a single dye as between different sets ofmicroparticle, varying the chemical nature of the dye (using differentdyes, or different combinations of dyes), or both.

A homogenous ternary solvent mixture according to the present inventionfor the preparation of dyed microparticles, particularly fluorescentlydyed microparticles, is schematically illustrated in FIG. 1( a). Solvent#1 is a strong solvent for both the dye and the polymer from which themicroparticle is formed. Solvent #2, also referred to herein as the“tuning solvent”, is a weak solvent or non-solvent for the dye and thepolymer. In a preferred embodiment, Solvent #2 is an aqueous solvent,preferably water. Solvents #1 and #2 are either immiscible or partiallymiscible with respect to each other. A third solvent, Solvent #3, is aweak solvent or non-solvent for the dye and polymer, but serves as aco-solvent with respect to Solvents #1 and #2 in that it is misciblewith both Solvents #1 and #2. In a preferred embodiment, Solvent #3 isan alcohol.

The prior art “swelling” methods of microparticle dye incorporation arelimited by the narrow range of choices of available solvents for dyes ofinterest, generally requiring the use of cross-linked particles. Theseprior art methods involve identifying a solvent of choice in which thedye is soluble over a range of concentrations, and preparing a dyesolution of desired concentration. Then, the dye solution is contactedwith the polymer microparticles for a period of time so as to permit thedye to penetrate into in the microparticles.

Prior art swelling methods of fluorescent particle production sufferfrom limited dye solubility in the dye bath. Even when dye solubility isnot an issue, the low partition coefficient of many dyes for the polymerrequires a large excess of valuable fluorescent dye, which is lost. Incontrast, the present invention produces microparticles of very high dyecontent, even from poorly soluble dye/solvent formulations. This aspectof the invention reflects the fact that the dye in the bath may becompletely depleted by solvent tuning.

In contrast to prior art solvent swelling based methods, the dyeincorporation method of the present invention may be used with equalefficacy for the dyeing of non-cross-linked as well as cross-linkedparticles. By “cross-linked” as describing a polymer comprising amicroparticle is meant a polymer in which chains are joined together toform a three-dimensional network structure. Cross-linking can be carriedout during the polymerization process by use of a cross-linking agent,that is, an agent which has two or more groups capable of reacting withfunctional groups on the polymer chain. Cross-linked polymers may alsobe prepared by the polymerization of monomers with an averagefunctionality greater than two.

The invention thus provides, for the first time, dye-loadedmicroparticles that are composed of a non-cross-linked polymer. This isa significant improvement because highly cross-linked particles areoften very difficult to synthesize. Furthermore, unlike many prior artparticle-dyeing methods that rely on intense mixing to achieveuniformity in dye staining of the microparticles, the present methodrequires only mild agitation. The mild agitation is required merely tokeep the particles suspended. This is a significant improvement overprior art methods because the intense mixing of those methods requiresspecialized equipment and is difficult to scale up.

Polymer cross-linking generally restrains swelling of microparticlesformed from cross-linked polymers, and also prevents penetration of thedye into the particle. As a result, the dye is restricted to a thinouter layer of the microparticle, and limits the dye loading. Theability to utilize non-cross-linked polymers as the microparticlematerial allows, for the first time, the production of dyed polymermicroparticles that are characterized by a substantially uniform dyedistribution throughout the volume of the microparticle. By“substantially uniform” is meant that the stained particle produces asymmetric and unimodal fluorescent intensity profile under conditions offluorescent imaging. In contrast, a surface-stained particle (where thefluorescent agent is confined to the surface, or to a shallow regionclose to the surface) produces a symmetric but bimodal fluorescentintensity profile.

In some circumstances, it may be desirable to obtain a controllednon-uniform dye distribution in the microparticle. Less than completedye penetration can be achieved by removing the microparticle from thestaining bath before the microparticle phase dye has reached equilibriumwith the liquid phase dye. The extent of dye penetration is determinedby the microparticles' incubation time in the staining bath. Removal ofthe particles from the bath prior to equilibration results in asymmetric but bimodal fluorescent intensity profile. The shape of theparticle fluorescence intensity profile, in particular the location ofintensity peaks, is a function of the pre-equilibration incubation time.Thus, microparticle incubation time in the staining bath provides afurther dimension for microparticle encoding. Microparticle sets ofvaried fluorescence intensity profiles may be produced, using the samedye but by varying the microparticle incubation times in the stainingbath. Multiple dyes may be utilized to provide even greater encoding.

According to the present invention, the amount of dye incorporated intothe microparticles is controlled by adjusting the ratio of dye withrespect to polymer, and by adjusting the composition of the dye bath. Inparticular, and in contrast to prior art methods, the volume fraction ofone constituent of the ternary solvent system, namely the tuningsolvent, is conveniently varied so as to control the partitioning of thedye between the solvent and the polymer comprising the microparticles.Unlike prior art methods, the invention provides considerably greaterflexibility in dye selection and solvent system formulation.Considerably greater control of dye partitioning is achieved using amulti-constituent solvent system. As elaborated below, the method of theinvention takes advantage of an exponential dependence of the partitioncoefficient K on solvent composition and thereby attains greater controlof dye incorporation than is achievable by the prior art “swelling”methods. Rather than tuning the partition coefficient, prior art methodssimply vary the initial dye concentration in the bath, and therebyachieve a proportional variation in the dye content of the microparticle(the proportionality constant being the partition coefficient K).

FIG. 2 contrasts the solvent tuning method of the present invention inthe context of varying dye partitioning into polymer microparticles withthe swelling method of the prior art. Points X₁ ⁽¹⁾, X₂ ⁽¹⁾ and X₃ ⁽¹⁾,each represent the mass of dye in the solvent system divided by thevolume of the microparticles contained in the system. Points Z₁ ⁽¹⁾, Z₂⁽¹⁾ and Z₃ ⁽¹⁾ each represent the corresponding concentration of dyeincorporated into microparticles The level of dye incorporation into themicroparticles is linearly related (line P_(o)) to the mass of dye inthe solvent system, the slope of the line being a function of thepartition coefficient K. It will thus be appreciated that the prior artsolvent-swelling methods confine the trajectories available for thepreparation of stained microparticles to the XZ plane. Multiplesub-populations of dyed microparticles are obtained only by explicitlyvarying the solvent bath initial dye concentration (for a given numberof microparticles to be stained) to produce a corresponding proportionalvariation in the level of dye incorporated into loading of theparticles.

In contrast, the present invention introduces solvent composition as anew variable to control the process of producing a multiplicity ofstained microparticles. It may be appreciated from a consideration ofFIG. 2 that the present invention provides for an entire additionaldimension of parameter space (Y) for the preparation of stainedmicroparticles by permitting trajectories within a 3-d parameter space(XYZ). For example, starting from the compositions Z₁ ⁽¹⁾, Z₂ ⁽¹⁾ and Z₃⁽¹⁾, distinct particle sub-populations displaying well-defined andpredictable levels of dye incorporation Z₁ ⁽²⁾, Z₂ ⁽²⁾, Z₃ ⁽²⁾ areprepared by following the non-linear operating curves shown in FIG. 2.At any fixed solvent composition (fixed Y) Z and X are related to eachother in a linear fashion (line OP₁, for a solvent composition with thevolume fraction of tuning solvent=φ).

Accordingly, any point in the three-dimensional parameter space XYZ ofFIG. 2 may be approached along a multiplicity of trajectories. In turn,each trajectory permits the preparation of multiple sub-populations ofstained microparticles in a predictable manner. The methods of thepresent invention, by operating in a regime governed by thermodynamicequilibrium and providing quantitative expressions for thesetrajectories, permit the rational design of protocols for thepreparation of multiple sub-populations of stained particles.

Without wishing to be bound by any theory, the operation of the presentinvention for controlling the partitioning of a solute (dye) between aliquid continuous phase and a dispersed solid phase (microparticles) maybe described by the following mathematical relationships.

The equation Z=G(X, Y) governs the transformation of the system from afirst state, {X₁, Y₁, Z₁} to a desired second state {X₂, Y₂, Z₂},wherein the concentration of the solute (dye) in the dispersed(microparticle) phase (Z) is a function of the concentration of solute(X) and solvent composition (Y). The desired second state {X₂, Y₂, Z₂}is selected from a multiplicity of possible such second statesaccessible from the given first state, by adjusting X and Y in aprescribed fashion in accordance with the equation Z=G(X, Y). Therelationship is governed by the variables (X), (Y) and (Z), which aredefined as follows:X=C ^((S)){φ^((S))/φ^((P))}Y={φ ^((P))/φ^((S))}Z=C^((P))wherein:

φ^((P))={V^((P))/(V^((P))+V^((S)))}=volume fraction of particle phase

φ^((S))=(1−φ^((P)))=volume fraction of the solvent phase

C^((S))=concentration of solute in solution phase at equilibrium

C^((P))=concentration of solute in particle phase at equilibrium

V^((P))=volume of particle phase

V^((S))=volume of solvent phase.

The distribution of the solute (dye) between the particle phase (P) andthe solvent phase (S) is governed by the partition coefficient, K:K={N ^((P)) /N ^((S))}wherein N^((P)) is the number of solute molecules in the particle phaseat equilibrium, and N^((S)) is the number of solute molecules in thesolvent phase at equilibrium.

Thus, the value of the partition coefficient K in state 1 is given asK ₁ =N ^((P)) /N ^((S)) ₁and the value of the partition coefficient K in state 2 is given asK ₂ =N ^((P)) ₂ /N ^((S)) ₂.

Applying the following mass balance equations,N^(T) ₁=N^(T) ₂ for the total amount of solute  (1)N ^((P)) ₁ +N ^((S)) ₁ +ΔN ^((S)) ₁ =N ^(T) ₁ for the total number ofsolute in state 1  (2)N ^((P)) ₂ +N ^((S)) ₂ =N ^(T) ₂ for the total number of solute in state2  (3)wherein N^(T) _(i) is the total amount (number) of solute molecules instate i, and ΔN^((S)) _(i) is the number of additional solute moleculesadded to the solvent phase in state i, results in the followingiterative equations:X ₂={(1+K ₁)/(1+K ₂)}X ₁+{1/(1+K ₂)}ΔX ₁  (4)Y ₂ =Y ₁ +ΔY ₁  (5)Z ₂={(1+K ₁ ⁻¹)/(1+K ₂ ⁻¹)}Z ₁+{1/(1+K ₂ ⁻¹)}ΔX ₁  (6)

In the special case of maintaining the variable Y constant, that is,transforming state 1 into state 2 solely by addition of solute so thatK₁=K₂, the following equation is obtained:Z ₂ =Z ₁+{1/(1+K ⁻¹)}ΔX ₁  (7)Further, the dependence of Z on X and Y is obtained in the form Z=K(Y)X.The dependence of the partition coefficient, K, on Y reflects the factthat addition of tuning solvent to the solvent phase diminishes thecapacity of solvent to dissolve the solute and therefore causesredistribution of solute into the available dispersed phase.Specifically, experimental data and analysis support the followingfunctional formula:K=a exp(bY)  (8)where a and b are constants determined from the analysis of data such asthose presented in FIG. 6 d.

Accordingly, the present invention provides an explicit set ofprescriptions to effect the desired state transformation. Specifically,an entire series of second states may be produced by transformation of asingle first state.

The above transformation equations, particularly in their iterative formas provided herein, facilitate the automated production of collectionsof dye-modified microparticles using a personal computer and a standardautomated pipetting instrument (“robot”) to dispense requisite meteredaliquots of dye or other solute as well as tuning solvent. A softwareapplication, developed in any standard programming language such asBASIC or C, is used to evaluate the iterative transformation equations.The program computes requisite aliquots of dye and tuning solvent. Thepipetting robot is accordingly controlled to meter and dispense theserequisite aliquots using a standard laboratory instrument controlinterface such as a GPIB protocol and a standard software developmentenvironment such as LabView (National Instruments). For example, using amaster batch as disclosed herein, sets of stained microparticles arereadily prepared by such a system by executing the steps of dispensingone or more aliquots of the master batch to produce a first state of thesuspension, computing requisite amounts of dye and dye solvent to attaina desired second state of the suspension, dispensing said requisiteamounts of solute and dye solvents, and permitting the transformation tooccur. These steps are repeated as desired.

Dyeing of functional group-modified microparticles by prior art sellingmethods may adversely affect the integrity of the functional group. Asdemonstrated by Example 28, below, functional group-modified particlesmay be dyed according to the practice of the present invention withoutloss of functional group integrity.

It may be appreciated that one of ordinary skill in the art may utilizereadily available information to select microparticle chemistries,solvents and dyes in accordance with the solubility parameters describedherein, for practicing the present invention.

It will also be apparent from the description of the process of theinvention that any polymer may be used to provide the polymer particlesprovided a stable dispersion of the polymer particles is available orcan be made. The material may comprise a homopolymer or copolymer, thelatter term meant to include not only polymers formed of two monomerunits, but also polymers formed of three or more monomer units,sometimes termed “terpolymers”. Hydrophobic polymers are preferred.Polymers comprising monomers of the vinyl class, that is, monomerscontaining the vinyl group, are particularly preferred, mostparticularly the styrene group. One group of preferred polymers includespolystyrene or polystyrene copolymers containing from about 50% to about100% by weight styrene monomer units. The polymer optionally may becross-linked or uncross-linked. In one embodiment, the microparticle isformed of polystyrene cross-linked with 1% divinylbenzene, based on theweight of the microparticle. In another embodiment, the microparticlecomprises styrene/methacrylic acid copolymer containing from about 0.6to about 1% methacrylic acid, based on the weight of the microparticle.

Suitable polymeric materials include, by way of example and not by wayof limitation, polymers of the following monomers:

acrylic acid, or any ester thereof, such as methyl acrylate, ethylacrylate, propyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate orglycidyl acrylate;

methacrylic acid, or any ester thereof, such as methyl methacrylate,ethyl methacrylate, propyl methacrylate, butyl methacrylate, laurylmathacrylate, cetyl methacrylate, stearyl mathacrylate, ethylene glycoldimethacrylate, tetraethylene glycol dimethacrylate, glycidylmethacrylate or N,N-(methacryloxy hydroxy propyl)-(hydroxy alkyl) aminoethyl amidazolidinone;

allyl esters such as allyl methacrylate;

itaconic acid, or ester thereof;

crotonic acid, or ester thereof;

maleic acid, or ester thereof, such as dibutyl maleate, dioctyl maleate,dioctyl maleate or diethyl maleate;

styrene, or substituted derivatives thereof such as ethyl styrene, butylstyrene or divinyl benzene;

monomer units which include an amine functionality, such as dimethylamino ethyl methacrylate or butyl amino ethyl methacrylate;

monomer units which include an amide functionality, such as acrylamideor methacrylamide;

vinyl-containing monomers such as vinyl ethers; vinyl thioethers; vinylalcohols; vinyl ketones; vinyl halides, such as vinyl chlorides; vinylesters, such as vinyl acetate or vinyl versatate; vinyl nitriles, suchas acrylonitrile or methacrylonitrile;

vinylidene halides, such as vinylidene chloride and vinylidene fluoride;

tetrafluoroethylene;

diene monomers, such as butadiene and isoprene; and

allyl ethers, such as allyl glycidyl ether.

Particularly preferred homopolymers and copolymers comprisingvinyl-containing monomers include polystyrene, poly(methylmethacrylate), polyacrylamide, poly(ethylene glycol),poly(hydroxyethylmethacrylate), poly(vinyltoluene) andpoly(divinylbenzene).

Suitable polymeric materials may include, by way of example and not byway of limitation the following polymers: polyoxides, such aspoly(ethylene oxide) and poly(propylene oxide); polyesters, such aspoly(ethylene terepthalate); polyurethane; polysulfonate; polysiloxanes,such as poly(dimethyl siloxane); polysulfide; polyacetylene;polysulfone; polysulfonamide; polyamides such as polycaprolactam andpoly(hexamethylene adipamide); polyimine; polyurea; heterocyclicpolymers such as polyvinylpyridine and polyvinyl pyrrolidinone;naturally occurring polymers such as natural rubber, gelatin, cellulose;polycarbonate; polyanhydride; and polyalkenes such as polyethylene,polypropylene and ethylene-propylene copolymer.

The polymeric material may contain functional groups such ascarboxylates, esters, amines, aldehydes, alcohols, or halides thatprovide sites for the attachment of chemical or biological moietiesdesirable to enhance the utility of the particles in chemical orbiological analyses. Methods for preparing microparticles from suchpolymers are well known in the art. Representative procedures forpreparing microparticles as well as cross-linked microparticles are setforth in the Preparative Examples, below.

The methods of the present invention also may be applied to the stainingof core-shell microparticles. Core-shell microparticles comprise acentral core of one or more core polymers and a shell of one or moreshell polymers containing the core. The polymer shell may be formed byany polymer-coating technique. Core-shell morphology isthermodynamically favored if the shell-forming polymer exhibits higherpolarity, or lower interfacial tension than does the core-formingpolymer. Core-shell morphology also is favored if the volume fraction ofthe shell-forming polymer is greater than that of the core-formingpolymer. Thus, synthesis of core-shell particles is performed at ashell/core weight ratio greater than 1. In certain embodiments, the corepolymer is hydrophobic and the shell polymer is relatively hydrophilicand carries functional groups of interest.

Copolymers of styrene and a monomer more hydrophilic than styrene (e.g.,methacrylic acid) are preferred for the core polymer over polystyrenehomopolymer. The comonomer serves to decrease the hydrophobicity of thecore and to render it more compatible with the hydrophilic shellpolymerization compositions.

Within these constraints, any monomer or combination of monomers may beselected as the shell polymer. A mixture of vinyl monomers is preferred.According to one embodiment of the invention, a monomer mixture ofmethyl methacrylate as the major constituent, and hydroxyethylmethacrylate and methacrylic acid as minor constituents, is used to forma shell over a polystyrene or modified polystyrene core. One suchmonomer mixture is composed of, by weight, about 6% hydroxyethylmethacrylate, from about 5% to about 20% methacrylic acid, the balancebeing methyl methacrylate. These monomers are more hydrophilic thanpolystyrene.

Microparticle size may be chosen appropriately for the intended end use.Typically, particles will range in size from about 0.1 to about 100microns in diameter, more typically from about 0.5 to about 50 microns,even more typically from about 2 to about 10 microns. Preferably, themicroparticles are “monodisperse”, that is, microparticles in a set havea narrow size range, preferably displaying a coefficient of variation ofthe mean diameter (“CV”) of no more than about 5%.

Microparticles may be rendered magnetically responsive by incorporationof an appropriate magnetic material, before or after staining, accordingto well-known procedures. According to one such method, particles areimpregnated with a ferrofluid, such as a ferrofluid prepared accordingto Example 19. By “magnetically responsive” is meant the ability tochange location or orientation in response to application of a magneticfield.

The dye may comprise any dye that imparts a visual or machine-observablecolor or fluorescence. The color or fluorescence may be detectable withthe naked eye or with the aid of a microscope or other opticalinstrument. The preferred fluorescent dyes are styryl dyes, such asp-bis(o-methylstyryl)benzene; pyromethane dyes such as fluorescent greendye 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-pentanoicacid, succinimidyl ester and the fluorescent orange dye4,4-difluoro-5-(2-thienyl)-4-bora-3a,4a-diazo-s-indacene-3-propionicacid-succinimidyl ester; and coumarin dyes such as methoxycoumarin.Preferred are those fluorescent dyes having emission wavelengths in therange from about 400 nm to about 1000 nm. When more than one dye isused, the dyes can be selected so that they have substantially differentabsorption spectra, emission spectra or emission lifetimes.

According to one embodiment, the microparticle comprises a polystyrenepolymer or copolymer and the dye is a hydrophobic dye. Possible solventsare selected, for example, from Table 1.

TABLE 1 Candidate solvents for polystyrene microparticle and hydrophobicdye combination Solvent #2: Weak solvent Solvent #1: Good solvent ornon-solvent for Solvent #3: Weak solvent or non- for hydrophobic dye andhydrophobic dye and solvent for hydrophobic dye and polystyrenemicroparticle polystyrene microparticle polystyrene microparticle (DyeSolvent) (Tuning Solvent) (Co-Solvent) methylene chloride water Acetonechloroform lower alcohols, especially tetrahydrofuran methanol, ethanoland isopropanol dioxane cyclohexane benzene toluene butylacetate lowerchlorinated aliphatic hydrocarbons.

A representative system utilizing a polar, water-soluble dye is composedof poly(ethylene oxide) microparticles in a ternary solvent systemcomprising water as Solvent #1; hexane as Solvent #2; and dioxane asSolvent #3.

From the solvents listed in the above table and a standard solventmiscibility chart, several ternary solvent systems may be designed for acombination of polystyrene polymer or copolymer and hydrophobic dye inaccordance with the present invention. For example, a hydrophobic dyesolute may be dissolved in a liquid phase consisting of a homogeneousternary mixture of water (tuning solvent, Solvent #2), alcohol(co-solvent, Solvent #3) and dichloromethane (dye solvent, Solvent #1)and contacted with a solid polymeric phase consisting of polystyrene orpolystyrene copolymer microparticles. The dye partition coefficient, K,governing the relative abundance of dye in the polymeric phase vs. thatin the ternary solvent mixture, increases as the volume fraction oftuning solvent in the liquid phase increases, and correspondinglydecreases as the volume fractions of dye solvent or cosolventdecreases). In addition to the water/alcohol/dichloromethane ternarysystem disclosed, other representative ternary systems include, forexample, water/acetone/methylene chloride.

The invention has been described for purposes of illustration ascontaining one each of Solvent #1, Solvent #2 and Solvent #3, thecharacteristics of which have been described above. However, it ispossible to practice the invention by including more than one solvent ineach category. For example, the solvent mixture may contain a singlesolvent of type #1, two solvents of type #2, and a single solvent oftype #3.

The microparticles of the invention may be functionalized to includechemical or biological entities such as, for example, nucleic acids andfragments thereof, including aptamers, proteins, peptides, and smallorganic molecules. The attachment of such molecules can be performedusing processes known in the art, for example, a covalent couplingreaction. See, e.g., G. T. Hermanson, Bioconjugate Techniques (AcademicPress, 1996) and L. Illum, P. D. E. Jones, Methods in Enzymology 112,67-84 (1985), the entire disclosures of which are incorporated herein byreference. These entities can be selected depending on the assay ofinterest. Examples of such assays are disclosed in PCT/US01/20179 andU.S. Pat. No. 6,251,691, which are incorporated herein by reference intheir entirety.

One embodiment of the method of the invention is diagrammaticallysummarized in FIG. 1( b). Microparticles are first incubated with asolvent solution comprising Solvent #2 and Solvent #3 in the presence ofoptional stabilizers to form a suspension. The function of thestabilizer is to prevent the destabilization of the suspension ofmicroparticles. Representative stabilizers include polymers,particularly polymeric alcohols, such as polyvinyl alcohol; polymericoxides, such as polyethylene oxide; polyvinyl polymers, such aspolyvinylpyrrolidone; poly acids, such as polyacrylic acid. Otherrepresentative stabilizers include ionic surfactants, such as sodiumdodecylsulfate and Aerosol OT; and non-ionic surfactants, such aspolyoxyethylene sorbitan monolaurate and polyethyleneglycoltert-octylphenyl ether. The concentration of the stabilizer may rangefrom about 0% to about 2%, by weight of the solvent/microparticlesuspension.

The suspension is preferably subjected to slow agitation. The incubationis typically conducted at room temperature, but higher or lowertemperatures may be utilized so long as the integrity of themicroparticles is not adversely affected, and the solvent compositionremains stable. The incubation is conducted to permit the optionalstabilizer to adsorb onto the microparticles. The requisitepre-incubation time will vary according to the composition of thesolvents and microparticles, and may be selected accordingly.

Following the above first incubation step, dye solubilized in dyesolvent (Solvent #1) is then added to the microparticle suspension.Sufficient dye should be added to ensure incorporation of dye to thedesired level in order to generate a detectable dye signal. Theincubation is carried typically out at room temperature, but higher orlower temperatures may be used. The second incubation is conducted topermit the dye solvent (Solvent #1) to penetrate the microparticles.

It has been found that the combination of a first step of incubatingparticles with a mixture of a Solvent #2 and a Solvent #3, and optionalstabilizers(s), followed by a second step of adding dye dissolved inSolvent #1, substantially decreases the need for intense mechanical oracoustic mixing during the dyeing step, as required by prior artprotocols. The particles require only mild agitation during the dyeingprocess in order to keep them suspended. This is a significantimprovement because intense mixing requires specialized equipment and isdifficult to scale up.

According to one embodiment, the concentration of dye in themicroparticle suspension is selected in the range from about 1 μg/g ofparticles to about 100 μg/g of particles, based upon the weight of theparticle suspension. Concentrations below and above this range may beappropriate in some applications depending on the composition of thesolvent solution and microparticles.

An amount of tuning solvent is then slowly added to favor partitioningof dye into the microparticles, while the suspension is slowly agitated.The volume fraction, φ, of tuning solvent is selected so as to attain adesired endpoint composition along a trajectory in FIG. 2.

The tuning solvent should be added at a controlled rate to maintainphase stability in the suspension. By “phase stability” is meant acondition characterized by the presence of an essentially homogeneousmixture of solute (dye) and liquid phase. Under a condition of phasestability, the dye remains dissolved in the solution phase while beingincorporated into the microparticles. The dye does not precipitate outof the solvent. Phase stability is further characterized by the absenceof liquid-liquid phase separation.

According to prior art methods, complete uptake of the dye into themicroparticle phase must be obtained so that the dye loading of themicroparticles may be derived with certainty, based upon the initialdye, amount in the solvent bath and the microparticle volume. In suchmethods, the precise level of dye loading should be known to ensure thatthe dye signal emitted is within the dynamic range of instrumentsutilized for detecting that signal. An accurate determination of the dyeloading is particularly important when a library of particles is to beconstructed, and different particle sets are to be distinguished bydifferent loadings of the same dye. Thus, the level of dye incorporationmust be monitored until no more dye is apparent in the suspensioncontinuous phase, signaling that substantially all the dye introducedinto the system has been take up by the microparticle phase.

According to the present invention, the amount of dye incorporated intothe microparticles is precisely controlled by modulating the amount oftuning solvent added to the microparticle suspension, as determined bythe iterative equations discussed above. A pre-selected level of dyeloading may be delivered with certainty, even in the absence of completepartitioning of the dye into the microparticle phase. By “completepartitioning” with respect to a solute (e.g., dye) is meant the statecharacterized by essentially complete uptake of the solute from theliquid phase to the microparticle phase, and the essentially completeabsence of dye from the liquid phase. Thus, it is not necessary tomonitor the status of dye migration from the liquid phase of themicroparticle suspension into the microparticle phase to ensure that alldye in the system has been taken up by the microparticles, as completedye uptake is not critical to the control of the dye loading.

The suspension of microparticles in the staining bath should beincubated for a period of time so as to provide substantially uniformpartitioning of dye into the microparticles.

To complete the process, the microparticle suspension is centrifuged,and the microparticles are optionally washed and resuspended in asuitable buffer, typically an aqueous buffer containing optionalsurfactants. The resulting microparticles comprise a set of dyedparticles containing a pre-determined, specific amount of dye thatpermits the identification of particles from a given set.

According to one embodiment, sub-populations of polymer microparticlescontaining different levels of incorporated dye may be produced inparallel fashion. Pre-calculated amounts of tuning solvent are added toseparate aliquots of microparticle suspension pre-incubated in dyesolution. In accordance with the present invention, the level of dyepartitioning into the microparticles is determined by the final volumefraction of tuning solvent and the initial dye concentration in thesuspension. This approach is illustrated in FIG. 3( a), where thevarious pre-incubated aliquots are denoted as B_(n). The respectiveamounts of tuning solvent introduced into each aliquot are representedas S_(n), and the corresponding resulting sub-populations of particlesin each batch are denoted as F_(n)(S_(n)).

According to another embodiment of the invention, sub-populations ofpolymer microparticles containing different levels of dye are producedin a serial fashion. From a master-batch microparticle suspension in dyesolution, aliquots are withdrawn at different elapsed times duringcontinuous or semi-continuous addition of the tuning solvent to themicroparticle suspension in dye solution. In “semi-continuous” additionof tuning solvent, the process is momentarily interrupted, for example,to permit the operation of removing a sample of microparticles from thebatch. Fractions of the suspension F_(n)(S_(n)) collected at successiveelapsed times (t_(Fn)), contain correspondingly differing amounts ofsolvent Sn and yield multiple sub-populations of stained particles fromthe same master batch. These sub-populations corresponding to levels ofdye incorporation will produce correspondingly differing fluorescenceintensities. This approach is illustrated in FIG. 3( b) wherein [B]denotes the pre-incubated master-batch, to which a continuous stream ofthe tuning solvent is fed. F_(n)(S_(n)) are the fractions of themicroparticle suspension collected from the master-batch at successivelapsed times, t_(Fn), respectively.

Alternatively, when a specified time (t_(F)) has lapsed, the continuousaddition of tuning solvent is interrupted, and the suspension of dyedmicroparticles is divided into two or more aliquots for adjustment offinal dye content by solvent tuning. A selected amount of tuning solvent(δS_(n)) is added to each aliquot so as to produce different levels ofdye incorporation in at least two aliquots, each said level beingdetermined by the total amount of tuning solvent added during solventtuning and to the initial dye concentration. It may be appreciated thatthe selected amount of tuning solvent added to each aliquot may comprisezero in one or more aliquots, provided that a non-zero amount of tuningsolvent is added to at least one of the aliquots. Thisserial-followed-by-parallel processing approach is illustrated in FIG.3( c).

Other variations of serial and parallel processing are possible. FIG. 3(d) illustrates a process combining serial and parallel processing,employing both solvent tuning and direct adjustment of dyeconcentration. As shown in FIG. 3( d), a continuous stream of tuningsolvent is fed into master-batch [B]. Fractions of microparticlesuspension F_(n)(S_(n)), respectively containing S_(n) amounts of thetuning solvent, are collected. Each fraction is then subjected to aseparate labeling step, initiated, for example, by addition of a thirddye, D_(n), permitting discrimination of microparticles in previouslyidentical aliquots. The labeling steps respectively involving dyes D₁,D₂, . . . D_(n) produce distinguishable sub-populations F_(m)(S_(m),D_(n)) from each aliquot.

Microparticles from each aliquot comprise a set of particles containinga pre-determined, specific amount of one or more fluorophores (orchromophores) permitting the identification of particles from a givenset.

The method of the present invention may be adapted to provide a libraryof combinatorially encoded microparticles by sequential addition ofsolutions of distinguishable fluorescent dyes. The microparticles areencoded in accordance with any one of a variety of available codes,including binary codes. Preferably, the microparticles are encoded witha binary encoding method that permits in-situ decoding, such as themethod of WO 98/53093, the entire disclosure of which is incorporatedherein by reference.

The practice of the invention is illustrated by the followingnon-limiting examples.

PREPARATIVE EXAMPLE 1 Non-Cross-Linked Polystyrene Homopolymer Particles

A 100-ml round bottom glass flask, equipped with a reflux condenser, N₂inlet-outlet adapter and an agitator, was placed in a jacketed oil bath.The flask was charged with a solution of 0.9475 g of polyvinylpyrolidone(Aldrich, average molecular weight about 29, 000) in 43.3 ml of ethylalcohol (Aldrich, 200 proof, anhydrous, 99.5%) and 18.95 g styrene. Inorder to remove free oxygen, the system was purged with N₂ for one halfhour under mild agitation (50-70 rpm). Then, the temperature was raisedto 70° C. and the agitator speed to 350 rpm. Polymerization of styrenemonomer was initiated by adding 10 ml of a solution of 2.4 wt %2,2′-azobisisobutyronitrile in ethanol. After 17 hours of reaction, thesystem was cooled to room temperature. Monodisperse polystyreneparticles having a volume average diameter of 4.1 μm were obtained. Themonomer conversion efficiency was 96.4% and the solids content of thefinal latex was 27.9%.

PREPARATIVE EXAMPLE 2 Non-Cross-Linked Copolymer Particles

The same procedure as Preparative Example 1 was used to prepare apolystyrene copolymer containing 3% methacrylic acid, by reacting 10.5 gof a mixture of styrene and methacrylic acid monomers (3 wt. %methacrylic acid monomer, based upon the total monomer weight).Monodisperse particles were obtained. The final conversion was 95.7%,particle diameter 3.2 μm, and the latex contained 15.9% solids. Thecopolymer particle had a parking area of 2.45 μm²/COOH group.

PREPARATIVE EXAMPLE 3 Cross-Linked Copolymer Particles

A 100-ml round bottom flask equipped with a reflux condenser, N₂inlet-outlet adapter, and agitator was placed in a jacketed oil bath.The flask was charged with 1.5 g of polyvinylpyrolidone (as inPreparative Example 1), 0.475 g of sodium dioctyl sulfosuccinate(Aldrich, 98%), 53.5 ml of ethyl alcohol (Aldrich, 200 proof, anhydrous,99.5%), 9.405 g styrene and 0.095 g divinylbenzene (Aldrich, mixture ofisomers, 80% purity). After removing the free oxygen by purging N₂ for30 min., the temperature was raised to 70° C. The polymerization wasstarted by adding 0.095 g of 4,4′-azobis(4-cyanovaleric acid) (Aldrich,75%) dissolved in 10 ml of ethanol. After 27 hours, the reaction wasstopped by cooling to room temperature. Monodisperse particles wereobtained. The monomer conversion was 93% and the particle volume averagediameter was 1.6 μm.

PREPARATIVE EXAMPLE 4 Non-Cross-Linked Core-Shell Particles

A 100 ml three-neck round bottom flask, equipped with a mechanicalstirrer, an inlet-outlet N₂ purge and a condenser was placed in athermostatted water bath at 70° C. To the flask 5.48 g of a latexcontaining 12.3 wt. % polystyrene monodisperse particles having adiameter of 3.15 μm was added. To this latex a solution 0.009 g sodiumdodecyl sulfate and 0.007 g sodium bicarbonate dissolved in 43.3 ml ofdistilled deionized water was added. The suspension was agitated at 100rpm and allowed to reach 70° C. under N₂ purge. When the temperature ofthe reaction mixture was stable, 0.0068 g of potassium persulfate in 0.5ml of distilled deionized water was added. Immediately following thisthe reaction was started by feeding a mixture of 0.676 g of a mixture of74% methyl methacrylate, 6% hydroxymethyl methacrylate and 20%methacrylic acid at a rate of 0.01 ml/min with a syringe pump. After thecompletion of feeding (1.2 h) the reaction was allowed to proceed underagitation for 2 additional hours at 70° C. The reaction was thenquenched by adding 0.0068 g of hydroquinone in 1 ml water and cooledrapidly to room temperature. A latex of 2.75 wt. % solids havingmonodisperse core-shell particles of 3.32 μm diameter was obtained. Thesurface carboxyl group parking area was 1.52 Å²/group.

EXAMPLE 1 Synthesis of Fluorescent Green Non-Cross-Linked Microparticles(Dye/Polymer=0.334 mg/g)

A 25 ml three neck round bottom glass flask was charged with 0.05 g ofcleaned (1 ml ethanol, three rounds of centrifugation (6500 rpm at roomtemperature) and redispersion) and non-cross-linked copolymer particles,added as 0.312 ml of the latex as prepared in Preparative Ex. 2. To theparticle suspension, 1 ml sodium dodecyl sulfate solution (“SDS”) (0.75wt. %), 1.5 ml poly(vinyl alcohol) (Aldrich, molecular weight85,000-146,000, hydrolyzed grade 87-89%) as a 0.1% water solution, and4.75 ml of ethanol were added. To this mixture, 0.0835 ml of adichloromethane (Aldrich, 99.9%) solution containing 0.0167 mg offluorescent green dye, Bodipy FL C5, SE(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-pentanoicacid, succinimidyl ester, Mw=417.22, Molecular Probes) was added.Finally, 10.6 ml of distilled deionized water was added and the mixtureagitated for another 2 hours. The whole mixture was then transferred toa 50 ml plastic centrifuge tube and centrifuged at 4500 rpm for 1 min.The supernatant was removed and the pellet containing the colored beadswas washed three times with 2 ml of ethanol, and finally resuspended in2 ml of 0.2% SDS solution. The ratio of green dye to polymer was 0.334mg/g. The intensity and the uniformity of the green fluorescence weredetermined using a Nikon fluorescence microscope with a charge-coupledevice (CCD) camera and image acquisition software. The results areshown as a scatter plot in FIG. 4 The ordinate of each spot representsthe green fluorescence intensity associated with a single particle; datawere not corrected for background signal/noise.

EXAMPLE 2 Preparation of Green Fluorescent Cross-Linked Microparticles(Dye/Polymer=1.667 mg/g)

This example is similar to Example 1, except that the polymer particleshad a cross-linked structure, and the scale of the experiment wasincreased four-fold. A 2 ml latex emulsion containing 0.2 g ofcross-linked core-shell particles (Bangs Laboratories, Inc., 3.2 μm, 10%solids, 12.5% divinylbenzene) was cleaned of emulsifier by adding 1 mlethanol and centrifuging at 6500 rpm for 2 min. This operation wasrepeated 3 times. The cleaned polymer particles were transferred to a100 ml round bottom flask filled with 6 ml of an aqueous solution of 1.0wt. % polyvinyl alcohol, 4 ml of a aqueous solution of 0.75 wt. % SDSand 19 ml ethanol. To this mixture, 1.5 ml of CH₂Cl₂ containing 0.3334mg of fluorescent green dye, Bodipy FL C5, SE, was added. After this, 53ml of distilled deionized water was added. The particle suspension wasthen transferred to a rotary evaporator and the solvents were removedunder vacuum (26.5 Hg inches) while the temperature was graduallyincreased to 40° C., then to 56° C., and finally to 63° C. for removalof the organic solvents. The concentrated colored suspension wascollected and centrifuged at 6500 for 2 min. and the supernatantdiscarded. The microparticle pellet was washed by three rounds ofcentrifugation and resuspended with 5 ml ethanol. Finally, the cleanedcolored beads were resuspended in 2 ml SDS 0.2 wt. % to a concentrationof about 10% solids. The ratio of green dye to polymer was 1.667 mg/g.The intensity and the uniformity of the green fluorescence weredetermined using a Nikon fluorescence microscope with attached CCDcamera and image acquisition software. The results are shown as ascatter plot in FIG. 5. The ordinate of each spot represents the greenfluorescence intensity associated with a single particle; data were notcorrected for background signal/noise.

EXAMPLE 2A Library of Fluorescent Green Dye-Encoded Cross-LinkedMicroparticles (Initial Dye/Polymer Concentration=0.833 mg/g)

The procedure of Ex. 1 was followed, except that the green fluorescentdye amount in the CH₂Cl₂ solution was 166.67 mg and the water feed ratewas 21 ml/h. During the course of water addition, four separatefractions were withdrawn at a time interval of 30 minutes apart. Theserial run thus generated five different populations of coloredparticles (one fraction was collected before starting the water feed)with each population having a distinct mean fluorescence intensity,which was a function of the amount of water added at the fraction, waswithdrawn. The particles were analyzed according to the proceduredescribed in Ex. 1. The green fluorescence intensity value measured foreach type of colored particles is presented in Table 1a. In Table 1a,and elsewhere herein, “a.u.” means arbitrary units.

TABLE 1a Green fluorescence intensity values for samples withdrawn inEx. 2A. Particle Fraction # Added water/polymer (ml/g) fluorescenceintensity (a.u.) 1 52.5 283.4 2 105.0 1396 3 157.5 3623 4 210.0 13061 5262.5 14658

EXAMPLE 2B Library of Dual-Colored Non-Cross-Linked MicroparticlesEncoded with Green and Orange Fluorescent Dyes (Initial Dye/PolymerConcentration: Green Dye/Polymer=Orange Dye/Polymer=0.75 mg/g)

A set of distinguishable dual-colored non-cross-linked particles encodedwith green and orange fluorescent dyes was produced by invoking theserial solvent tuning method of the present invention, using thefollowing initial dye concentrations: Green fluorescent dye/polymer=0.75mg/g; orange fluorescent dye/polymer=0.75 mg/g. Three separate fractionswere collected according to the methodology of Example 2A above, exceptthat two fluorescent dyes were present in the initial pre-incubationsuspension. The amounts of water added until the time of fractionwithdrawal and the mean intensities of the different fractions ofmicroparticles collected are shown in Table 1b.

TABLE 1b Green and Orange Fluorescent Intensities from Ex. 2B (26.5 ml/hwater fed rate) Fed Green Orange water/ fluorescent fluorescent[Green]_(i)/polymer [Orange]_(i)/polymer polymer intensity intensityFraction # mg/g mmol/g × 10⁻³ mg/g mmol/g × 10⁻³ ml/g (a.u.) (a.u.) 10.75 1.79 0.75 1.69 265.0 1220 1388 2 0.75 1.79 0.75 1.69 265.0 70827798 3 0.75 1.79 0.75 1.69 265.0 11698  15479 

EXAMPLES 3-6 Analysis of Dye Partitioning in the Preparation of GreenFluorescent Cross-Linked Microparticles

Four separate green fluorescent-dyed particle preparations were preparedby a protocol as in Ex. 2, but on a smaller scale, comparable to thescale of Ex. 1. The respective amounts of water added were: 0.833 ml,1.74 ml, 5.31 ml and 10.59 ml, corresponding to water volume fractionsof 0.398, 0.463, 0.623 and 0.738, respectively. Following completion ofdye incorporation, the colored particles were centrifuged. Thesupernatant was saved in order to determine, the green dye remaining insolution. Accordingly, equal amounts of each of the four supernatantsolutions were diluted (24×) with alcohol and fluorescence spectra wererecorded. The emission intensity values (see Table 2) were used tocalculate the concentration of green dye remaining in each solutionaccording to the calibration curve of FIG. 6( a). The concentration ofgreen dye concentration incorporated into the particles was calculatedas the difference between the total initial amount of green dye in thereaction and that remaining in the supernatant. These values are shownin Table 2.

TABLE 2 Dye partitioning details Dye Dye bath Supernatant Supernatantdye Initial incorporated Measured water fluorescence conc. (from dye inparticles particle volume intensity calibration curve) conc. (μg/mg)fluorescence Ex. # fraction (a.u.) (μg/ml) (μg/ml) (calculated)intensity 3 0.39806 580.4 1.808 1.929 0.0208 135 4 0.4629 494.15 1.5391.72 0.0348 539 5 0.6227 270.1 0.841 1.205 0.0997 2909 6 0.73823 54.340.169 0.834 0.2639 7581

FIG. 6( b) shows the nonlinear variation of incorporated dye content asa function of water volume fraction.

The calculated incorporated green dye content also was correlated to theintensity of green fluorescence recorded by fluorescence microscopy fromstained particles. The results in FIG. 6( c) display a linearcorrelation.

FIG. 6( d) shows the partition coefficient of the dye, K, plottedagainst Y. The curve exhibits a characteristic exponential dependenceillustrating the fine-tuning of dye incorporation by way of modulationof the solvent composition in accordance with the present invention.

EXAMPLE 7 Cross-Linked Particles Containing Green Fluorescent Dye(Dye/Polymer=0.833 mg/g)

The procedure of Example 2 was followed, except that the green dyesolution in CH₂Cl₂ added to the particle suspension had a concentrationof 0.1 mg/ml, and the experiment was run at half the scale in a 50 mlflask. Accordingly, the amount of water added following the addition ofthe dye solution, was 1.765 ml. The colored particles were recovered andwashed by repeated centrifugation and re-dispersion.

EXAMPLES 8-10 Cross-Linked Particles Containing Green Fluorescent Dye

The procedure of Example 7 was followed to generate three additionalparticle sets, except that the water amounts were 5.3 ml, 10.6 ml and21.2 ml, respectively.

EXAMPLE 11 Cross-Linked Particles Containing Orange Fluorescent Dye(Dye/Polymer=0.334 mg/g)

The procedure of Example 7 was followed, except that the dye was orangeBodipy 558/568, SE(4,4-difluoro-5-(2-thienyl)-4-bora-3a,4a-diaza-s-indacene-3-propionicacid, succinimidyl ester, Mw=443.23, Molecular Probes). Theconcentration of orange dye in methylene chloride solution was 0.045mg/ml. The amount of water added following the addition of the dyesolution was 1.765 ml. The ratio of orange dye to polymer was 0.833mg/g. The concentration of orange dye in methylene chloride solution was0.045 mg/ml.

EXAMPLES 12-14 Cross-Linked Particles Containing Orange Fluorescent Dye

The procedure of Example 11 was followed to generate three additionalparticle sets, except that the respective amounts of water added were5.3 ml, 10.6 ml and 21.2 ml.

EXAMPLE 15 Cross-Linked Particles Containing 1:1 Molar Ratio of Greenand Orange Fluorescent Dyes (Green Dye/Polymer=0.150 mg/g; OrangeDye/Polymer 0.153 mg/g)

The procedure of Example 7 was followed, except that instead of one dye,an equimolar mixture of the green and the orange dyes of Examples 7-14were used. Specifically, the methylene chloride solution with the twodyes had a concentration of 0.20 mg/ml of the green dye and 0.212 mg/mlof the orange dye.

EXAMPLES 16-17 Cross-Linked Particles Containing Differing Amounts ofGreen and Orange Fluorescent Dyes

The procedure of Example 15 was followed to generate two additionalparticle sets, except that the respective amounts of water added were5.3 ml and 21.2 ml.

EXAMPLE 18 Cross-Linked Particles Containing 1:0.5 Molar Ratio of Greenand Orange Fluorescent Dyes (Green Dye/Polymer=0.150 mg/g; OrangeDye/Polymer=0.765 mg/g)

The procedure of Example 15 was followed, except that green and orangedyes were used in a molar ratio of 1:05. Specifically, the methylenechloride solution containing the two dyes had a concentration of 0.20mg/ml of the green dye and 0.10 mg/ml of the orange dye.

EXAMPLES 19-21 Cross-Linked Particles Containing Green and OrangeFluorescent Dyes

The procedure of Example 18 was followed to generate three additionalparticle sets, except that the respective amounts of water added were5.3 ml, 10.6 ml and 21.2 ml.

EXAMPLE 22 Cross-Linked Particles Containing 0.5:1 Molar Ratio ofFluorescent Green and Orange Dye (Green Dye/Polymer=0.150 mg/g; OrangeDye/Polymer=0.765 mg/g)

The procedure of Example 15 was followed, except that (a) the green andorange dyes were used in a molar ratio of 0.5:1 and (b) the amount ofwater added was 1.5 ml. The methylene chloride solution containing thetwo dyes had a concentration of 0.10 mg/ml of the green dye and 0.212mg/ml of the orange dye.

EXAMPLES 23-25 Cross-Linked Particles Containing Green and OrangeFluorescent Dyes

The procedure of Example 22 was followed to generate three additionalparticle sets, except that the respective amounts of water added were5.3 ml, 10.6 ml and 21.2 ml.

The fluorescence intensities of the dyed particle sets prepared inaccordance with Examples 7-25 are shown in Table 3.

TABLE 3 Green and Orange Intensities as a function of initial dyeconcentrations and added water amount Orange Green Amt. of dye Watervolume Intensity Intensity Example # (mg/g polymer) fraction (mean)(mean) 7 0.833 0.2626 — 980.184 8 0.833 0.5168 — 2638.53 9 0.833 0.6815— 7251.29 10 0.833 0.8106 — 13175.8 11 0.333 0.2626 655.519 — 12 0.3330.5168 1676.34 — 13 0.333 0.6815 6518.44 — 14 0.333 0.8106 12434.9 — 150.303 0.2626 1118.73 851.539 16 0.303 0.5168 2707.29 1989.1 17 0.3030.8106 7296 5575.43 18 0.227 0.2626 196.022 326.917 19 0.227 0.5168537.007 740.114 20 0.227 0.6815 1175.03 1686.59 21 0.227 0.8106 3140.375008.65 22 0.228 0.2324 261.14 176.635 23 0.228 0.5168 1087.38 439.54524 0.228 0.6815 3469.4 1162.34 25 0.228 0.8106 8659.33 2974.67

EXAMPLE 26 Construction of a Fluorescence-Encoded Microparticle Library

A library containing the nineteen fluorescent microparticle setsaccording to Examples 7-25 was constructed. Ten of the nineteen setswere pooled and a fluorecence image of the mixture was recorded using aNikon fluorescence microscope attached with a CCD camera and imageacquisition software, permitting recording of the green and orangefluorescence. Ten clusters corresponding to the ten sets in the pool areapparent in the scatter plot (cluster map) of FIG. 7 employinglogarithmic units of orange and green intensities on abscissa andordinate, respectively. Results of the analysis of the clusters aresummarized in Table 4.

TABLE 4 Cluster mean intensities and corresponding coefficients ofvariation Cluster details Orange Cluster # Intensity (mean) CV(%) GreenIntensity (mean) CV(%) 11 211.363 10.27 541.50 8.65 12 584.10 6.741309.52 6.93 13 1300.47 5.66 3154.83 6.49 14 3369.32 4.75 9083.26 5.4715 1228.41 5.81 1515.42 6.61 16 7699.52 4.53 9973.90 5.40 17 2868.525.28 3511.96 6.0 18 1242.01 5.95 825.846 6.93 19 4077.29 4.60 2386.05.54 20 9546.03 4.92 5627.64 5.79

EXAMPLE 27 Preparation of Encoded Magnetic Particles

A. Synthesis of Aqueous Ferrofluid

Stock solutions of 1M FeCl₃ in 1N HCl and 2M FeCl₂ in 1N HCl wereprepared. In a 100 ml glass bottle, 4 ml of 1M FeCl₃ and 1 ml of 2MFeCl₂ solution were combined 400 ml of deionized distilled water and 100ml of a 30 wt. % NH₄OH solution were mixed to give 500 ml of an about1.7 M solution of NH₃ in water. Fifty ml of the ammonia solution wasadded slowly to the glass bottle containing the iron salt solutionsunder vigorous agitation. Following completion of this step. 2 ml of a25 wt. % solution of tetramethyl ammonium hydroxide was added and thesolution sonicated for about 1 hr. Following this, the ferrofluid wasallowed to settle overnight under the influence of a magnetic field.Next, the supernatant was decanted and the precipitate washed withdistilled water. The iron oxide nanoparticle suspension in deionizedwater was homogenized and allowed to settle overnight under theinfluence of gravity. Following settling, the precipitate was discardedand the dark colored supernatant collected as the final ferrofluidsuspension.

B. Synthesis of Encoded Magnetic Particles

Colored polymer microparticles of identical dye content and about 3microns in diameter were prepared according to methods described hereinto comprise a polystyrene core and a methyl methacrylate (MMA),hydroxyethylmethacrylate (HEMA) and methacrylic acid (MAA) shell. Theparticles were dispersed in de-ionized water to give 1 ml of an about 1%suspension. A 50 microliter aliquot of the ferrofluid suspension wasthen added to the suspension. The suspensions were admixed withend-over-end rotation for 48 hours at room temperature. The resultantsolution was centrifuged at about 200 g for 10 minutes. A tan coloredparticle pellet was separated from the brownish red colored supernatantcontaining the excess nanopaticles. The supernatant was discarded andthe pellet resuspended in 1% SDS solution and centrifuged again. Thisstep was repeated two times and the pellet finally redispersed in PBSbuffer with 0.5% Tween-20. The 1 ml particle suspension was taken in astandard 1.5 ml standard Eppendorf tube and the tube was mounted on aPromega Multitube Magnetic Stand. Complete separation of the suspendedparticles (as a pellet on the wall of the tube) took place in about 10minutes.

EXAMPLE 28 Coupling of Avidin to Surface of Fluorescence-Encoded,Carboxyl-Functionalized Microparticles

The carboxylate-functionalized polymer microparticles prepared accordingto Preparative Example 4 were rendered fluorescent according to theprocedure of Example 1 to provide green fluorescent microparticles(dye/polymer=0.334 mg/g). In a 2 ml vial, an aliquot containing 10 mg ofthe green fluorescent microparticles was mixed with 1 ml 10 mM boratebuffer (pH=8.5). The particles were then separated by centrifugation andthe supernatant was siphoned off. Following this, the separated pelletwas washed two times in 0.1M MES buffer (pH=4.5) and finally resuspendedin 600 μl of the same. In a separate vial, 3 mg of Neutravidin (abiotin-binding protein, Pierce Chemicals, Rockford, Ill.) was dissolvedin 300 μl of the MES buffer and the solution slowly added to thesuspension of the polymer microparticles. The suspension was brieflysonicated using a probe sonicator. Following this, 150 μl of a1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (Aldrich-Sigma,Milwaukee, Wis.) (EDAC) solution (200 mg/ml) was added to the particlesolution. The mixture was allowed to react for 2 hours at roomtemperature, following which NeutrAvidin-functionalized polymermicroparticles were separated, washed once in coupling buffer, twice inborate buffer and finally resuspended and stored in storage buffer (PBSpH=7.4, 0.1% (w/v) BSA, 0.5%(w/v) Tween 20, 10 mM ethylene diaminetetraacetic acid (EDTA) and 0.02% (w/v) NaN₃) at 2-8° C.

EXAMPLE 29 Coupling of Avidin to Surface of Fluorescence-Encoded,Tosyl-Functionalized Microparticles

Commercially available cross-linked, tosyl-functionalized fluorescentcore-shell microparticles (Bangs Laboratories, Inc., 3.2 μm, 10% solids,12.5% divinylbenzene) were used in this example. The microparticlescontained a green fluorescent dye loading of about 0.3 mg dye/gmicroparticle. Two hundred microliters of a suspension containing 1% ofthe microparticles were washed three times with 500 μl of 100 mMphosphate buffer (pH 7.4) and resuspended in 500 μl of that buffer.Following this, 20 μl of 5 mg/ml NeutrAvidin was added and the reactionallowed to proceed overnight at 37° C. Following completion of theincubation, the functionalized particles were washed once with 500 μl ofPBS (pH 7.4) containing 10 mg/ml BSA, resuspended in 500 μl of thatbuffer and reacted for 1 hr at 37° C. to block unreacted sites on themicroparticle surface. Following the blocking step, the microparticleswere washed three times with 500 μl of PBS (pH 7.4) containing 10 mg/mlBSA and stored in 200 μl of PBS (pH 7.4) with 10 mg/ml BSA.

EXAMPLE 30 NeutrAvidin-Biotin Binding Assay Using FluorescentMicroparticles

One hundred microliters of the NeutrAvidin-functionalized fluorescentmicroparticles, containing 1% solids of Example 28, were placed in a 1.5ml vial and the suspension diluted with 900 μl of PBS containing 0.01%(w/v) of Tween-20 (PBST). The microparticles were mixed by vortexing andthen separated by centrifugation. The supernatant was aspirated off, andthe pellet resuspended in 980 μl of PBS. 20 μl of abiotin-Oligo(dT)₅-CY5.5 (oligo labeled with a fluorescent dyeCy5.5)(IDT, Coralville, Iowa) at a concentration (26.7 ng/ml) was addedand the mixture was incubated for 30 minutes at room temperature.Following this, the microparticles were separated and washed twice inPBST and resuspended in 1 ml of PBST. The microparticles were thenassembled on a chip and their surface fluorescence was determined as adirect measure of the amount of biotin-Oligo(dT)₅-CY5.5 bound to theNeutrAvidin-functionalized particles. The results displayed in FIG. 8show the biotinylated probe capture efficiency of two differentparticles (marked as samples) dyed using the method of the presentinvention and the capture efficiency of a non-dyed microparticle thatwas used as a positive control.

EXAMPLE 31 Hybridization Assay Using Fluorescent Microparticles

Biotinylated oligonucleotides with known base sequence were attached tothe fluorescence-encoded microparticles functionalized with NeutrAvidin(as prepared in Example 30) as follows. Fifty microliters of a solutioncontaining 1% of the NeutrAvidin-functionalized microparticles wasplaced in 0.1 ml reaction buffer (150 mM NaCl, 0.05 M EDTA, 0.5% bovineserum albumin, 0.5 mM Tris-HCl, and 100 mM sodium phosphate, pH 7.2)containing 0.4 μM biotinylated oligonucleotides and approximately 7×10⁵microparticles. The reaction mixture was incubated at room temperaturefor 30 minutes under vortexing. Upon completion of the reaction, theparticles were collected by centrifugation, washed three times with PBST(150 mM NaCl, 100 mM sodium phosphate, pH 7.2 with 0.05% Tween 20) andresuspended in 0.2 ml PBS (150 mM NaCl, 100 mM sodium phosphate, pH7.2.). The foregoing procedure can be utilized to couple anybiotinylated oligonucleotide of interest to NeutrAvidin-functionalizedparticles.

One microliter of a 10 μM solution of a synthetic target (5′-/CY5.5/SEQID NO:1/-3′) in de-ionized water was diluted with 19 μl of 1×TMAC (4.5 Mtetramethyl ammonium chloride, 75 mM Tris pH 8.0, 3 mM EDTA, 0.15% SDS)to a final volume of 20 μl. Two types of oligonucleotide-functionalizedfluorescent microparticles were assembled into planar arrays on siliconsubstrates. The first microparticle type was functionalized with amatched probe sequence 5′-Biotin/(TEGspacer)/SEQ ID NO:2/-3′ The secondmicroparticle type was functionalized with a mismatched probe sequenceBiotin/(TEGspacer)/SEQ ID NO:3/-3′). Twenty microliters of the synthetictarget was added to the substrate surface and the substrate was placedin a 53° C. heater for 15 minutes under shaking at 30 rpm. The slide wasthen removed from the heater the target solution was aspirated. Thesubstrate was washed once with 1× TMAC at room temperature. Followingthis, 10 μl of 1× TMAC was placed on the substrate surface, covered witha glass cover-slip and the fluorescence intensity of the array recordedusing the instrumentation described before. The results in FIG. 9 showthat the hybridization was specific.

EXAMPLE 32 Immunoassay Using Fluorescent Microparticles

Commercially available cross-linked, tosyl-functionalized fluorescentcore-shell microparticles (Bangs Laboratories, Inc., 3.2 μm, 10% solids,12.5% divinylbenzene) were used in this example. The microparticlescontained a green fluorescent dye loading of about 0.3 mg dye/gmicroparticle. One ml of PBST (PBS pH 7.4 containing 0.1% Tween-20) and50 μL of a 1% suspension of the dyed tosylate-functionalizedmicroparticles (0.5 mg) were combined in an eppendorf tube and mixedwell by vortexing. Following this, the suspension was centrifuged at7500 rpm for 2 min. and the supernatant decanted. The operation wasrepeated once with 1 mL of PBST and once with 1 mL of PBS.Microparticles were finally resuspended in 1 mL of PBS. A pre-calculatedamount of anti-TNF-α antibody (R&D Systems), at a concentration of 50 μgprotein/mg microparticles, was added, and the suspension was incubatedovernight at room temperature under end-over-end rotation. Themicroparticles were then washed and resuspended in 1 ml ofblocking/storage buffer (0.1M PBS pH 7.4 containing 0.1% BSA, 0.1% Tween20 and 0.1% NaN₃). Ten microliters of the antibody-functionalizedmicroparticle suspension were placed in a 1.5 mL Eppendorf tube. Theparticles were washed twice with 1 mL of PBST and once with 1 mL of PBS(pH7.2). Thirty microliters of a stock solution of Cy5.5-labeled goatanti-mouse IgG was diluted by adding 1470 μL of PBS (1:50). Five hundredmicroliters of this solution was transferred to the microparticlesuspension and the antibody-binding reaction incubated for 60 min. atroom temperature under end-over-end mixing. Following incubation, theparticles were washed twice with 1 mL of PBST and then resuspended in 10μL of PBS. A planar array of microparticles was then assembled onsilicon substrate for analysis as in Example 31. An average Cy5.5intensity of 6,500 was recorded using the conditions and instrumentationdescribed before.

All references discussed herein are incorporated by reference. Oneskilled in the art will readily appreciate that the present invention iswell adapted to carry out the objects and obtain the ends and advantagesmentioned, as well as those inherent therein. The present invention maybe embodied in other specific forms without departing from the spirit oressential attributes thereof and, accordingly, reference should be madeto the appended claims, rather than to the foregoing specification, asindicating the scope of the invention.

1. A method for generating a plurality of microparticle populations ofdifferent dye concentrations wherein dye is loaded into polymermicroparticles to generate said microparticle populations, comprising:(a) providing: (i) at least one first solvent in which the dye and themicroparticle polymer are soluble; (ii) at least one tuning solvent inwhich the dye and the microparticle polymer are not or only weaklysoluble, said first and tuning solvents being immiscible or at mostpartially miscible; (iii) at least one third solvent in which the dyeand the microparticle polymer are not or only weakly soluble, said thirdsolvent being miscible with the first and tuning solvents; (b) forming aplurality of suspensions of said polymer microparticles, each in adesignated volume of a mixture including the tuning solvent and thethird solvent; (c) adding to said polymer microparticle suspension asolution including the dye dissolved in said first solvent to form ahomogeneous solution in the mixture of the first, tuning and thirdsolvents; and (d) increasing the uptake of dye into the polymermicroparticles in at least one of said populations relative to anotherof said populations by decreasing the solvency of the mixture of thefirst, tuning and third solvents for the dye.
 2. The method according toclaim 1 wherein the solvency of the mixture is decreased by addingtuning solvent.
 3. The method according to claim 1 wherein tuningsolvent is added continuously or semi-continuously to the microparticlesuspension to cause, respectively, a continuous or semi-continuousdecrease in the solvency of the mixture of the first, tuning and thirdsolvents for the solute, and an increase in the uptake of solute intothe polymer microparticles, but without causing precipitation of thesolute, in said at least one of said populations.
 4. The methodaccording to claim 3 further including the step of removing at least oneportion of said microparticles from the suspension at a time intervalduring the course of said tuning solvent addition to providemicroparticle sets which differ in dye concentration.
 5. The methodaccording to claim 3 wherein the microparticle suspension is incubatedfor a period of time.
 6. A method of producing dyed polymermicroparticles comprising: (a) providing: (i) at least one first solventin which the dye and the microparticle polymer are soluble; (ii) atleast one tuning solvent in which the dye and the microparticle polymerare not or only weakly soluble, said first and tuning solvents beingimmiscible or at, most partially miscible; (iii) at least one thirdsolvent in which the dye and the microparticle polymer are not or onlyweakly soluble, said third solvent being miscible with the first andtuning solvents; (b) forming a suspension of said polymer microparticlesin a designated volume of a mixture comprising at least one tuningsolvent and at least one third solvent; (c) adding to said polymermicroparticle suspension a solution comprising the dye dissolved in saidfirst solvent whereby the dye is taken up by the microparticles toprovide a master-batch suspension of microparticles characterized by afirst concentration of said dye in the microparticles; (d) creating twoor more aliquots from said microparticle master-batch suspensioncontaining selected added amounts of tuning solvent, wherein the addedtuning solvent increases the amount of dye partitioning to the polymermicroparticles in said aliquots; and (e) incubating the polymermicroparticle suspension aliquots for a period of time so that theamount of dye partitioning to the microparticles, for a given initialdye concentration in the dye solution, is controlled by the amount oftuning solvent added to the microparticle suspension aliquots.
 7. Themethod according to claim 6 wherein the step of creating said two ormore microparticle suspension aliquots containing selected added amountsof tuning solvent comprises dividing the microparticle suspensionmaster-batch into two or more aliquots, and adding selected amounts oftuning solvent to said aliquots.
 8. The method according to claim 6wherein the step of creating said two or more microparticle suspensionaliquots containing selected added amounts of tuning solvent comprisescontinuously or semi-continuously adding tuning solvent to themicroparticle suspension master-batch and removing at least one portionof said master-batch at a time interval following said tuning solventaddition to form two or more microparticle suspension aliquotscontaining selected added amounts of tuning solvent.
 9. The methodaccording to claim 6 wherein the dye is a fluorescent dye.
 10. Themethod according to claim 9 wherein the dye is a hydrophobic dye. 11.The method according to claim 9 wherein the dye is selected from thegroup consisting of styryl dyes, pyrromethane dyes, coumarin dyes, andcombinations thereof.
 12. The method according to claim 6 wherein themicroparticles comprise a hydrophobic polymer.
 13. The method accordingto claim 12 wherein the polymer is a homopolymer or copolymer comprisinga vinyl-containing monomer.
 14. The method according to claim 12selected from the group consisting of homopolymers or copolymers ofstyrene, methyl methacrylate, acrylamide, ethylene glycol,hydroxyethylmethacrylate, vinyltoluene, divinylbenzene, and combinationsthereof.
 15. The method according to claim 14 wherein the polymer ispolystyrene or copolymer thereof containing at least 50% by weightstyrene monomer units.
 16. The method according to claim 15 wherein thepolymer is a styrene/methacrylic acid copolymer.
 17. The methodaccording to claim 13 wherein the polymer is cross-linked.
 18. Themethod according to claim 6 wherein the microparticles have a diameterof from about 0.1 to about 100 microns.
 19. The method according toclaim 18 wherein the particles are monodisperse.
 20. The methodaccording to claim 6 wherein the concentration of dye present in themicroparticle suspension formed by contacting microparticles with saiddye solution is from about 10 μg/g to about 100 μg/g, based upon theweight of the microparticle suspension.
 21. The method according toclaim 6 comprising a solvent wherein the first solvent is selected fromthe group consisting of methylene chloride, chloroform, tetrahydrofuran,dioxane, cyclohexane, benzene, toluene, butylacetate, lower chlorinatedaliphatic hydrocarbons, and combinations thereof; the tuning solvent iswater; and the third solvent is selected from the group consisting ofacetone, lower alcohols, and combinations thereof.
 22. The methodaccording to claim 21 wherein the first solvent is methylene chloride ordichloromethane, and the tuning solvent is water.
 23. The methodaccording to claim 22 wherein the third solvent is alcohol.
 24. Themethod according to claim 6 wherein the microparticle is a core-shellmicroparticle comprising a central core comprising one or more corepolymers surrounded by a shell comprising one or more shell polymers.25. The method according to claim 24 wherein the core polymer comprisesa copolymer of styrene and a monomer more hydrophilic than styrene. 26.The method according to claim 24 wherein the core polymer comprisesmethacrylic acid.
 27. The method according to claim 6 wherein at least aportion of the microparticles are magnetically responsive.
 28. Themethod of claim 6 wherein the solvency of the microparticle suspensionfor the solute is decreased by adding tuning solvent.
 29. The method ofany of claims 1, or 6 wherein an emulsion is not formed in said polymermicroparticle suspension.